This invention relates to arrays of infrared detector elements of pyroelectric or ferroelectric material, and in particular 2-dimensional arrays of such elements which may be used, for example, for thermal imaging purposes requiring an inexpensive image sensor.
Arrays of infrared detector elements mounted on a support are well known, in which each element comprises a capacitor formed by a body of pyroelectric or ferroelectric material between a front electrode and a back electrode at respective front and back major faces. The back major face of the body faces the support. The elements have individual electrical connections to their back electrode from which there are derived electrical signals which differ as the temperature of the body changes in response to incident infrared radiation. By having these individual back connections, the signals derived from the individual detector elements can be fed to, for example, separate amplifiers via separate outputs. At their front major face, all the elements of the array may have a common electrical connection to their front electrodes, or groups of the elements may have common connections.
A linear array of such infrared detector elements of high performance but inexpensive to manufacture is disclosed in the paper entitled "The application of heat-collector fins to reticulated pyroelectric arrays" by A. A. Turnbull, presented in Cannes, France in Nov. 1985 and published in the Proceedings of SPIE (Society of Photo-Optical Instrumentation Engineers, USA) Vol. 588, Recent developments in materials and detectors for the infrared (1985), pages 38 to 43. The individual back connections of these pyroelectric elements are provided by a pattern of conductors carried by the support which in the form disclosed in this SPIE paper is a polyimide membrane. Each element has an infrared-collection area in the form of a metal/dielectric/metal sandwich structure (termed a "heat fin") of larger lateral dimensions than both the individual pyroelectric body of each element and the electrical connection to its back electrode. This fin forms a peripheral portion of each element which overhangs and is separated vertically from an underlying part of the support. The whole contents of said SPIE paper by A. A. Turnbull are hereby incorporated as reference material in the present specification.
As described in said SPIE paper, the infrared active area of each element is defined with high precision by the array of heat fins which are formed by thin film techniques and which are spaced apart by gaps of, for example, 20 .mu.m (micrometres), while reticulating the pyroelectric material into individual bodies which are spaced apart by gaps of, for example, 50 .mu.m. This permits the gaps between the bodies to be formed by sawing in a convenient manner through the pyroelectric material. The provision of the overhanging heat fin also improves the Noise Equivalent Power (NEP) of the pyroelectric detector element, as described in more detail in published United Kingdom Patent Application GB-A-2 100 058, the whole contents of which are hereby also incorporated as reference material in the present specification. In the case of pyroelectric materials the electrical signals result from electrical charges generated at the major faces as a result of the spontaneous dipole moment of the material changing with change in temperature. However, such detector element structures with heat fins may also be used with ferroelectric material operated near its Curie temperature so that its dielectric constant is a strong function of temperature. In this case the electrical signals may be derived as a difference in the magnitude of a voltage across the capacitor detector element at different temperatures.
It is known to form 2-dimensional arrays of pyroelectric and other ferroelectric infrared detector elements by mounting the array on a silicon circuit substrate comprising, for example, a switching matrix of MOS transistors for addressing the individual detector elements. The electrical connections to the back electrodes of the detector elements are formed in a vertical configuration by, for example, electro-deposited metal rods, solder bumps, or metallised bores between the detector elements and the silicon circuitry. Examples of such structures are described in, for example, published United Kingdom patent application GB-A-2 030 023, U.S. Pat. No. US-A-4 532 424 and US-A-4 072 863, and published European patent application EP-A-0 173 368. The electrical signal generated by each detector element arises from a change in its temperature in response to the infrared radiation collected by that element, so that the thermal isolation of the individual elements is important. In the known 2-dimensional array structure it is not easy to obtain good thermal isolation of the individual elements both from the underlying silicon circuit which has a large thermal capacitance and from each other due to the thermal paths present between the elements and their back connections. Thus, in general, the detector elements in these known 2-dimensional arrays have inferior thermal isolation as compared with fully reticulated elements mounted on a support of electrically and thermally insulating material, for example on a polyimide film as described in the said SPIE paper and in GB-A-2 100 058. Furthermore, the mechanical reliability of the small cross-section electrodeposited rods and solder bumps as connections to a silicon circuit is not entirely satisfactory for a volume manufacturing process, and this approach can be expensive, especially for 2-dimensional arrays with a small number of detector elements, for example a square array of 6.times.6 elements or less. Thus, it is more expensive than using thin or thick-film techniques with wide conductor tracks. However, in order to facilitate the use of thick-film techniques in a reliably reproduceable manner in a volume manufacturing process, it is desirable for the conductor tracks to be wider than the spacing normally desired between detector elements in a 2-dimensional array.